A. Field Of The Invention
The present invention relates to a free piston shock tube/tunnel, and more particularly, to a free piston shock tube/tunnel having means to control and optimize the holding time and thus the test time for a particular shock tube structure, without resulting in a corresponding increase in viscous losses.
B. Description Of The Prior Art
Free piston shock tube/tunnels have existed since the 1950's. During operation, free piston shock tube/tunnels are able to generate a shock wave of extremely high pressure and high temperature at a test site for a desired duration or test time for the purpose of providing test conditions for aerodynamic studies. Free piston shock tube/tunnels are principally used to test the aerodynamic conditions relating to rocket nose cones, space re-entry vehicles, and other hypersonic aircraft.
In general, a free piston shock tube/tunnel includes an elongated, generally cylindrical compression tube containing a compression or driver gas such as helium. The compression tube is closed at one end by a diaphragm with a preselected rupture pressure and includes a compression piston adapted for movement from the piston starting end forward toward the diaphragm end. Connected to the diaphragm end of the compression tube is an elongated shock tube having a test end remote from the diaphragm and being filled with a low pressure driven gas such as ambient air. When the piston is moved from the piston starting end of the compression tube toward the diaphragm end, the compression gas within the compression tube is compressed, thus causing the diaphragm to rupture. The rupturing of the diaphragm causes a volume of the compressed compression gas to pass through the ruptured diaphragm and into the connected shock tube generating the shock wave. The shock wave compresses the driven gas during movement through the shock tube, thereby creating the desired test conditions at the test site. In the case of a shock tunnel, the shock wave compressed gas is further processed through a nozzle expansion to the final test site.
The piston in a conventional free piston shock tube/tunnel is driven by compressed gas introduced behind the piston. During the compression movement of the piston toward the diaphragm, the gas in the compression tube can be compressed to pressures as high as 2,000 atmospheres or greater. This in turn can generate a shock wave in the shock tube which can create test conditions in the driven gas with temperatures as high as 12,000K and pressures as high as 3,000 atm.
In prior art free piston shock tube/tunnels, the shock tube is of generally cylindrical construction having a single, constant diametrical dimension less than that of the diametrical dimension of the compression tube. In typical free piston shock tunnel structures, the diameter of the compression tube is at least about three times greater than the diameter of the shock tube.
Despite the utilization of free piston shock tube/tunnels for nearly 40 years, and despite continuing studies for the purpose of more fully understanding the operation, and optimizing the performance, of free piston shock tube/tunnels, their general construction has not changed significantly. A typical free piston shock tunnel is disclosed in Patent Cooperation Treaty publication number WO 89/02071 by Raymond Stalker. Published studies relating to the performance and operation of free piston shock tube/tunnels include an article entitled "Pressure Losses In Free Piston Driven Shock Tubes" by N. W. Page and R. J. Stalker in Shock Tubes and Waves (14th International Symposium on Shock Tubes and Shock Waves), August, 1983 at page 118 and an article entitled "The Piston Motion In A Free Piston Driver For Shock Tubes And Tunnels" by Hans G. Hornung at GALCIT, California Institute Of Technology, 1988.
A concept introduced in the above identified article by Hans G. Hornung is that of holding time. As defined by Mr. Hornung and generally accepted in the art relating to free piston shock tube/tunnels, holding time is the time interval over which the pressure in the compression tube deviates from its maximum value by 10% or less. This, of course, occurs near the end of the piston travel and shortly after rupturing of the diaphragm. It is known that the holding time and the test time (the duration of desired test conditions at the test site) are related. Increasing the holding time will generally result in a corresponding increase in the test time at the test end of the shock tube. It is also known that for a given design of compression tube and piston, longer holding times can be generated by decreasing the diameter of the shock tube. As the shock tube diameter is decreased, however, the viscous losses increase. Viscous losses are those losses resulting from frictional forces acting between the shock tube wall and the flow behind the shock wave as it moves through the shock tube. An increase in viscous losses tends to decrease the test time. Thus, there is a limit to how much the test time can be increased by reducing the diameter of the shock tube.
Further, because of the generally fixed and unalterable nature of currently existing free piston shock tunnels, the cost associated with such shock tunnels and their use is extremely high. For example, if one wishes to alter the test conditions, such as by reducing or increasing the diameter of the shock tube, significant rework is needed. This results in extremely high costs and significant down time. Accordingly, the ability of current free piston shock tubes/tunnels to be used for a wide variety of different tests and applications is extremely limited.
Accordingly, there is a need in the art for an improved free piston shock tube/tunnel construction by which the holding time can be increased, thereby allowing longer test time, without a corresponding increase in viscous losses. There is also a need for a free piston shock tube/tunnel of a given construction which has applicability for a variety of different test environments and which can be quickly and easily altered to provide different desired holding times and test conditions.